``Buffett has typically hung his stock market bullishness, or, in a few rare instances, bearishness, on the ratio that compares the total value of all publicly traded companies with gross domestic product. When the market cap of US stocks collectively equal about 80% of GDP, as it did in early 2009, Buffett has said it is time to buy. When the value grows to 130% of GDP, as it did in late 2007, it is time to sell. The problem now is that the measure crossed 130% in early 2017 (it also reached that threshold in late 2015, before stocks dropped), and the stock market has continued to rise. It is now close to 150%, even with the market's recent wobbles.''